The preparation of crystalline silicoaluminophosphates is well known. U.S. Pat. No. 4,480,871 describes the preparation of crystalline, microporous silicoaluminophosphates by hydrothermal crystallization of silicoaluminophosphate gels containing a molecular structure forming template. SAPOs are members of a class known as non-zeolitic molecular sieves. The SAPO molecular sieve has a framework of AlO.sub.4, SiO.sub.4 and PO.sub.4 tetrahedra linked by oxygen atoms. The negative change in the network is balanced by the inclusion of exchangeable protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces of channels formed by the crystalline network enables SAPOs to be used as molecular sieves in a manner similar to crystalline aluminosilicates such as zeolites.
Accordingly, numerous microporous framework structures analogous to the aluminosilicate zeolites can be synthesized having an AlPO.sub.4 composition and have been called ALPOs. A modified family of materials has been made by the substitution of Si.sup.4+ for Al.sup.3+ and P.sup.5+ (SAPOs). Although the ALPO structures are neutral frameworks, the substitution of Si.sup.4+ for P.sup.5+ imparts a negative charge on the framework. By suitable choice of a cation, this can be translated into catalytic activity. However, alternate substitutions may be possible that may result in a disproportionately low exchange capacity. The exact nature of Si substitution into ALPO structures is complex and highly variable and may depend on both the topology of the ALPO/SAPO and the method of preparation. The result is that preferred catalysts may be made by a suitable choice of synthesis method. For example, SAPO-5 and SAPO-11 may be conventionally prepared in an aqueous solution or from microemulsions. The latter processes use hexanol and a cationic or neutral surfactant to a two-phase gel leading to the formation of a microemulsion.
The microemulsion process is a two-phase approach to preparing SAPOs attempts to reduce the amount of undesirable silica island formation by supplying the silicon from an organic phase to the aqueous phase at a low concentration during crystallization. The organic phase contains the organic solvent and organic silicon source, tetraethylorthosilicate, which is only slightly soluble in the aqueous phase. The aqueous phase is where crystallization occurs and contains the phosphorous and aluminum. It has been theorized that as the silicon is depleted from the aqueous phases by the growing SAPO crystals, it will be replenished from the organic phase, thereby forming a silicoaluminophosphate product having a more uniform distribution of silicon in the framework.
SAPOs have application for a wide variety of uses, for example as catalysts. In this regard, it is known that increasing Si concentration at first results in an increase in catalystic activity. However, increasing Si content beyond about 0.04 mole fraction in the framework, based on the total amount of silicon, aluminum, and phosphorous in the framework, provides no increase in activity, and may even lead to a decrease, depending on the specific distribution and clustering of the Si.sup.4+ substituent.
In that the distribution of Si in the SAPO framework affects catalytic activity, the catalytic activity of SAPOs therefore depends on both the global composition and the Si distribution. Accordingly, SAPOs are defined not only by chemical composition and X-Ray Diffraction, but also by .sup.29 Si MAS NMR spectra which define the Si distributions. On the basis of this last technique, it has been shown that when the SAPOs contain low amounts of Si, the silicon atoms are mostly isolated. However, when the Si content increases, Si islands start to appear, i.e., Si sites having silicon atoms and no aluminum atoms in neighboring lattice positions.
It would, therefore, be desirable to further increase silicon content in SAPO molecular sieves without forming undesirable silica islands in order to increase catalytic activity and selectivity.